Absorption and Emission Sensitivity of 2‐(2′‐Hydroxyphenyl)benzoxazole to Solvents and Impurities

2‐(2′‐Hydroxyphenyl)benzoxazole (HBO) is known for undergoing intramolecular proton transfer in the excited state to result in the emission of its tautomer. A minor long‐wavelength absorption band in the range 370–420 nm has been reported in highly polar solvents such as dimethylsulfoxide (DMSO). However, the nature of this species has not been entirely clarified. In this work, we provide evidence that this long‐wavelength absorption band might have been caused by base or metal salt impurities that are introduced into the spectral sample during solvent transport using glass Pasteur pipettes. The contamination by base or metal salt could be avoided by using borosilicate glass syringes or nonglass pipettes in sample handling. Quantum chemical calculations conclude that solvent‐mediated deprotonation is too energetically costly to occur without the aid of a base of an adequate strength. In the presence of such a base, the deprotonation of HBO and its effect on emission are investigated in dichloromethane and DMSO, the latter of which facilitates deprotonation much more readily than the former. Finally, the absorption and emission spectra of HBO in 13 solvents are reported, from which it is concluded that ESIPT is hindered in polar solvents that are also strong hydrogen bond acceptors.